Theiler's virus is a picornavirus responsible for a persistent infection of the central nervous system of the mouse, leading to a chronic demyelinating disease considered to be a model for multiple sclerosis. The leader (L) protein encoded by Theiler's virus is a 76-amino-acid-long peptide containing a zinc-binding motif. This motif is conserved in the L proteins of all cardioviruses, including encephalomyocarditis virus. The L protein of Theiler's virus was suggested to interfere with the alpha/beta interferon (IFN-␣/␤) response (W.-P. Kong, G. D. Ghadge, and R. P. Roos, Proc. Natl. Acad. Sci. USA 91:1796-1800, 1994). We show that expression of the L protein indeed inhibits the production of alpha/beta interferon by infected L929 cells. The L protein specifically inhibits the transcription of the IFN-␣4 and IFN-␤ genes, which are known to be activated early in response to viral infection. Mutation of the zinc finger was sufficient to block the anti-interferon activity, outlining the importance of this motif in the L protein function. In agreement with the anti-interferon role of the L protein, a virus bearing a mutation in the zinc-binding motif was dramatically impaired in its ability to persist in the central nervous system of SJL/J mice.Theiler's murine encephalomyelitis virus (TMEV) (or Theiler's virus), a member of the Picornavirus family, is a naturally occurring enteric pathogen of the mouse, responsible for central nervous system (CNS) infections (32). The neurovirulent strains (GD7 and FA) cause an acute lethal encephalomyelitis. The persistent strains (DA and BeAn) induce a biphasic disease after intracerebral inoculation of susceptible mice (18). After a mild encephalomyelitis lasting about 2 weeks, mice develop a chronic demyelinating disease, which serves as an experimental model of multiple sclerosis (for review, see references 8 and 25).TMEV can be recovered from the spinal cord white matter virtually lifelong, indicating that active viral replication occurs during persistence despite the host immune response. Viral persistence appears to be required to induce the chronic demyelinating disease, but the exact mechanisms involved in persistence are still poorly understood. Among the viral determinants of persistence identified, the capsid plays a crucial role, probably affecting the tropism of the virus in the CNS (2, 11, 22). However, viral factors allowing the virus to escape the host immune response could also play a pivotal role in establishing persistence.Antagonism of the innate immune response mediated by alpha/beta interferons (IFNs-␣/␤) is a common determinant of virulence (33). Indeed, IFNs-␣/␤ are cytokines produced by most cell types in response to viral infection. The antiviral action of IFNs is mediated by the activation of proteins, such as protein kinase R (PKR), the 2Ј-5Ј-oligodenylate synthetase, or the Mx proteins, known to interfere with the viral cycle (29).The genome of picornaviruses is translated as a long precursor polyprotein that undergoes autoproteolytic cleavage to yiel...

Theiler's virus is a neurotropic murine picornavirus which, depending on the strain, causes either acute encephalitis or persistent demyelinating disease. Persistent strains of Theiler's virus (such as DA) produce an 18-kDa protein called L* from an open reading frame overlapping that encoding the viral polyprotein. Neurovirulent strains (such as GDVII) are thought not to produce the L* protein, as the alternative open reading frame of these strains starts with an ACG codon instead of an AUG codon. However, we observed that both persistent and neurovirulent strain derivatives can produce two forms of the L* protein through unusual type II internal ribosome entry site-mediated translation. A full-length 18-kDa protein can be expressed from an ACG or an AUG initiation codon, whereas an N-terminally truncated 15-kDa product can be translated from a downstream AUG initiation codon. The expression of the 18-kDa form is required for efficient persistence of DA virus derivatives in the central nervous system.

No abstract

SummaryHuman herpesvirus 8 (HHV-8) infection is associated with the development of Kaposi's sarcoma and primary effusion lymphoma. The cloning of the HHV-8 genome into a bacterial artificial chromosome (BAC) allows researchers to mutate and identify the relative importance of HHV-8 genes essential for growth and replication in tissue culture systems. However, in vivo models to study the impact of such mutations are very limited. Consequently, the objective of this study was to determine whether cells carrying the HHV-8 BAC would form tumors when injected into mice, enabling the use of this model to assess the influence of viral gene mutation on tumorigenesis. To do so, 293T and 293T-E1 cells carrying recombinant HHV-8 were injected into SCID mice and tumor growth was analyzed. Our results clearly show that mice injected with 293T-E1 cells had a significantly higher tumor incidence level as well as increased tumor volumes and weights compared to mice injected with 293T control cells. Cells carrying the HHV-8 genome grew faster and more aggressively in SCID mice than control 293T cells, highlighting the oncogenic properties of HHV-8. The model presented could therefore be used for the identification of HHV-8 genes contributing to tumorigenesis in the context of the entire viral genome.